Method of stain-decorating glass-ceramics

ABSTRACT

Glass-ceramic articles having compositional inhomogeneities are provided with uniformly colored gray stain decorations by heat treating a palladium-containing staining material prior to application onto glass-ceramic articles which have been crystallized by a separate heat treatment, and then subjecting the glass-ceramic articles in contact with the staining material to another heat treatment to produce the stain decorations.

BACKGROUND OF THE INVENTION

The present invention relates to an improvement in the process for decorating glass-ceramic materials with patterned stains. More particularly, the invention relates to a mode of staining glass-ceramics whereby greater uniformity of coloration is achieved without causing the color of the stain to diverge from the customary gray or blue-gray.

Glass-ceramics are articles in which a major crystalline phase is dispersed in a glassy matrix, and which are individually produced by forming an article from a crystallizable glass and subsequently subjecting the glass article to heat treatment in order to induce crystallization within the body of the glass. The resulting glass-ceramic products are useful because of their relatively low coefficient of thermal expansion and high resistance to thermal shock, which lend the glass-ceramic articles to use in high temperature applications. A particular application for which the use of glass-ceramics has become popular is as cooking surfaces in domestic appliances such as stove tops and microwave oven shelves. Additional information regarding the basic nature of glass-ceramic compositions and methods of making may be found in U.S. Pat. No. 2,920,971 to S. D. Stookey and in U.S. Pat. No. 3,625,718 to R. W. Petticrew.

It is often desired to apply a decorative pattern to a glass-ceramic article, and in order to provide such a decoration with abrasion resistance, it is preferred to impart the decoration of the glass-ceramic by staining, whereby the coloration is contained within the surface portion of the articles. Staining is carried out by applying a stain composition to the crystallizable glass surface in the desired pattern, such as by silk-screening, and then maintaining the crystallizable glass at an elevated temperature to migrate stain-producing ions into the surface portion of the glass-ceramic. For domestic appliance applications, the preferred staining compositions include palladium as the staining agent, which usually produces a neutral gray or blue-gray colored stain without detrimentally affecting the luster of the glass-ceramic surface. Suitable palladium stains are commercially available in the form of palladium resinates in an organic solvent to which significant amounts of inert extender, such as titanium dioxide, may be added. Prior to this invention, crystallization and staining usually have been carried out simultaneously in one heat treatment step.

Occasionally, it is found that glass-ceramic articles stain-decorated in the customary manner with palladium are stained non-uniformly. That is, some areas of the stained pattern achieve a darker gray coloration than other areas of the stained pattern. These non-uniformly stained glass-ceramic articles were unsightly and had to be discarded. It has been theorized that the non-uniformity of the stained patterns was caused by inhomogeneities in the composition of the glass-ceramic articles arising from the melting and forming process. Glass-ceramic compositions require relatively high melting temperatures and are, therefore, prone to losing volatile constituents of the composition during melting, and their melting may be accompanied by leaching of elements from the refractory furnace walls into the melt, both of which lead to inhomogeneities in the product withdrawn from the furnace. It has been found that the darker gray areas in the non-uniformly stained patterns correspond to areas of the glass in which the zirconia concentration is slightly greater than that of the bulk of the glass-ceramic articles.

Various methods and compositions for stain-decorating glass-ceramics are disclosed in the following U.S. patents:

U.S. Pat. No. 3,313,644--J. W. Morrissey

U.S. Pat. No. 3,528,847--P. Grego et al.

U.S. Pat. No. 3,775,154--P. Grego et al.

U.S. Pat. No. 3,816,161--M. J. Buckley

U.S. Pat. No. 3,852,052--H. M. Demarest, Jr.

U.S. Pat. No. 3,940,531--H. M. Demarest, Jr.

SUMMARY OF THE INVENTION

It has now been found that crystallizable glass articles having inhomogeneities which would normally lead to non-uniform coloration when stain-decorated with palladium can be successfully provided with uniformly colored stained patterns by carrying out the crystallizing and staining steps in separate, sequential heat treatment steps. However, post-crystallization staining of glass-ceramics has been found to produce a color which deviates from the desired neutral gray or blue-gray. Therefore, another aspect of the present invention is the discovery that stained decorations having the standard gray color, as well as uniform color density, may be produced by heat treating the stain composition separately before it is applied to the crystallized glass-ceramic. In heat treating the stain material, it is subjected to sufficiently high temperatures to decompose the organic constituents of the composition, including the resinate radicals. After the heat treatment, the residue of the stain composition is reconstitued with a suitable vehicle to form a paste which may be silk-screened or otherwise applied to the crystallized glass-ceramic articles. The glass-ceramic articles with the reconstituted stain composition applied thereto are then subjected to a second heat treatment in accordance with the first aspect of the invention. After the second heat treatment it is found that a uniform, neutral gray stain is formed within surface portions of the glass-ceramic, despite the presence of inhomogeneities.

THE DRAWING

The drawing is a flow diagram which schematically depicts the steps of stain-decorating glass-ceramics in accordance with the present invention.

DETAILED DESCRIPTION

Crystallizable glass compositions and methods for producing glass-ceramic articles are known in the art and specific reference may be had to the aforementioned U.S. Pat. Nos. 2,920,971 and 3,625,718, the disclosures of which are hereby incorporated by reference.

In general, the crystallizable glass compositions of the present invention may be characterized as having essential inclusions of SiO₂, Al₂ O₃, and Li₂ O as crystal-forming constituents, ZnO as a melting aid, and TiO₂ or a mixture of TiO₂ and ZrO₂ as nucleating agents. The alkali metal content of the glass-ceramics is minimized, although a small amount of K₂ O is typically included. Small amounts of melting and fining aids, such as fluorine, chlorine, antimony, or arsenic may also be included. The presence of arsenic and/or antimony has also been found to have a beneficial effect on the staining process. An example of a crystallizable glass composition is as follows:

    ______________________________________                                                    Percent by Weight                                                   Ingredient   Range     Preferred Embodiment                                    ______________________________________                                         SiO.sub.2    67-71     70.22                                                   Al.sub.2 O.sub.3                                                                            18-21     19.21                                                   TiO.sub.2    1.4-5.0   2.15                                                    ZrO.sub.2    0-2.0     1.58                                                    Sb.sub.2 O.sub.3                                                                            0-1.0     0.38 (Sb.sub.2 O.sub.5)                                 As.sub.2 O.sub.5                                                                            0-1.0     0.01                                                    Li.sub.2 O   2.5-4.0   3.99                                                    Na.sub.2 O   0-1.0     0.30                                                    K.sub.2 O    0-1.0     0.27                                                    Cl.sub.2     0-0.2     --                                                      ZnO          0.5-2.0   1.59                                                    F.sub.2      0-0.5     0.24 (F.sup.-)                                          MgO          0-3.0     0                                                       CaO          0-4.0     0                                                       P.sub.2 O.sub.5                                                                             0-1.5     0                                                       ______________________________________                                    

A glass of the above preferred composition may be melted from the following batch ingredients:

    ______________________________________                                         Ingredient           Parts by Weight                                           ______________________________________                                         Silica               700                                                       Hydrated alumina     296                                                       Lithium carbonate    83                                                        Zinc zirconium silicate                                                                             31                                                        Zinc oxide           10.5                                                      Titanium dioxide     15.0                                                      Lithium fluoride     13.5                                                      Soda ash             4.0                                                       Lithium sulfate      6.0                                                       Potassium carbonate  2.5                                                       Antimony oxide       4.0                                                       Total                1165.5                                                    ______________________________________                                    

These materials may be melted on a continuous basis in a refractory melting chamber, from one end of which, in one embodiment, a ribbon may be withdrawn and formed into a flat sheet of glass by rolling in accordance with techniques similar to the plate glass method. Following forming, the glassy sheet is cooled and cut to the desired size. Optionally, the glassy sheet also may be ground and polished.

Crystallization of the formed glassy articles (e.g., plates) takes place in a heat treatment chamber into which the articles are placed at room temperature. The temperature is steadily raised over a period of several hours to about 1300° F. (700° C.) or higher, which temperature is maintained for several hours to initiate nucleation of crystallization sites within the bodies of the glassy articles. The temperature is then increased to about 1850° F. (1010° C.) to convert a major portion (at least 50 percent) of the glass to small, dispersed crystals. The crystal which first forms is beta-eucryptite, which is subsequently transformed by the heat treatment to beta-spodumene crystals. Preferably, crystallization is carried out until the article is about 98 percent by weight crystal phase, with 2 percent remaining as a glassy phase. In its final state, the crystalline phase is solid solution of beta-spodumene and silica.

A specific example of a preferred heat treating schedule is as follows:

Raise temperature from room temperature to 1100° F. (593° C.) over 3 hours;

Hold at 1100° F. (593° C.) for 2 hours;

Raise temperature to 1285° F. (696° C.) over 2 hours;

Raise temperature to 1325° F. (718° C.) over 1 hour;

Increase temperature to 1385° F. (752° C.) over 6 hours;

Hold at 1385° F. (752° C.) for 2 hours;

Increase temperature to 1400° C. (760° C.) over 2 hours;

Increase temperature to 1650° F. (899° C.) over 1.5 hours;

Hold at 1650° F. (899° C.) for 2 hours.

From this point crystallization is ordinarily completed by further increasing the temperature to 1850° F. (1010° C.) over 1.5 hours, holding at 1850° F. (1010° C.) for 3 hours and then cooling to room temperature over a period of about 3 hours. However, for purposes of the present invention, it is preferred to stop crystallization heat treatment after holding at 1650° F. (899° C.), even though conversion to the beta-spodumene crystal form is incomplete, since the subsequent heat treatment required for stain-decorating the glass-ceramic articles may be employed to carry crystallization to the desired point of completion. The result is a savings in thermal energy.

If the conventional palladium stain-decorating compositions are applied to crystallized glass-ceramics and subjected to elevated temperatures in the usual way in order to create stained patterns, it has been found that the resulting stain is not the desired neutral gray or blue-gray color, but is a tan or brown color. This discovery is the subject matter of a copending patent application Ser. No. 942,254, filed on even date herewith by V. Lupoi and assigned to the assignee of this application. In accordance with the present application, however, it is possible to produce gray colored stain patterns on glass-ceramics which have already been crystallized. This is accomplished by heating a conventional stain-decorating composition to a temperature above the decomposition temperature of the organic constituents of the composition, reconsitituting the residue of the heat treated stain composition by mixing with a suitable vehicle, applying the mixture in the conventional manner to crystallized glass-ceramic articles, and subjecting the articles to a second heat treatment to drive the stain into the glass surface. The relationship of the several heat treating steps of the present invention is graphically illustrated in the drawing.

A specific palladium glass colorant which may be used with the present invention is identified as "Dark Brown A-1454" by the manufacturer, Englehard Industries, Inc., Newark, N.J., and includes a palladium resinate as the active staining agent, along with a small amount of bismuth in an organic carrier. The colorant contains about 2.26 weight percent palladium and 0.42 weight percent bismuth. It should be noted that the type of colorants used for staining glass-ceramics are often not marketed primarily for such a use, but rather for producing surface decorations or conductive films on glass, ceramics, glass-ceramics, and other materials. But when in contact with glass-ceramics under the type of thermal conditions disclosed herein, stains rather than films are produced. About 50 grams of the colorant are mixed with about 450 grams of titanium dioxide extender, which is preferably in the anatase crystal form as taught in U.S. Pat. No. 3,816,161 to M. J. Buckley. The colorant and titanium dioxide may be mixed in a ball mill with a suitable liquid vehicle to establish the desired viscosity for use in the particular decorating technique to be employed. For use in a silk-screen decorating method pine oil is the preferred vehicle, for example: "Drakelene Oil" sold by Hercules, Inc., Drakenfeld Division, Washington, Pa. A viscosity of about 50,000 centipoise is typically considered suitable for silk-screening.

The composition described above is the form in which the stain is conventionally applied to crystallizable glass articles in the prior art practice. In the present invention, the stain composition is subjected to a separate heat treatment prior to application onto the glass-ceramics. A substantial quantity of the stain composition is placed in a vessel and heated in a well ventilated facility to boil off the organic vehicle at a slow rate to prevent splattering. The residue is ground to -200 mesh, placed on a tray and heated in a furnace to a temperature at least sufficient to decompose any organic compounds present including the resinate portion of the palladium compounds. Although temperatures considerably lower may be adequate, it is preferred to heat the stain composition to a temperature similar to the maximum temperature obtained in the crystallizing heat process, that is, about 1850° F. to 2000° F. (1010° C. to 1093° C. ) for about two hours. A silica tray with the powered material loaded to a depth of about one-half inch (one centimeter) has been found to be satisfactory for heat treating the stain material. The heat treated stain material is then reconsitituted with a suitable vehicle, such as pine oil, for application to the glass-ceramic articles by the particular decorating technique chosen, preferably silk-screening.

The crystallized glass-ceramic articles with the heat treated stain material applied thereto are then returned to the heating chamber where they are heated to a temperature sufficient to drive the stain-producing palladium ions into surface portions of the glass. In the case of the specific heat treatment schedule set forth above, wherein crystallization was stopped short of completion, the second heat treatment preferably consists of heating the heating chamber to a temperature of 1950° F. (1066° C.) over a period of about ten hours (or approximately the maximum rate of the heating chamber) and holding at 1950° F. (1066° C.) for one hour, after which the heat is turned off and the heating chamber is permitted to cool over a period of several hours. At the conclusion of the second heat treatment, the residue of the staining material is wiped from the surfaces of the glass-ceramic articles. The stain patterns which are formed are found to be a uniform neutral gray color closely matching that produced in glass-ceramics which are stain-decorated and crystallized simultaneously. The remainder of each crystallized article has an almost opaque, milk-white to grayish-white appearance. By employing other heat treatment schedules, glass-ceramics which are transparent or translucent may be produced.

It should be apparent that the procedure set forth above for heat treating the stain composition utilizes as its starting material the formulation prepared for silk screening merely for the sake of convenience, since that is the form in which the stain is readily available. But since the silk screening formulation contains substantial amounts of organic vehicle which is boiled off in the heat treatment, the starting material need not include any such inactive ingredients. Furthermore, since the resinate radicals of the palladium compounds are decomposed by the heat treatment, it appears that the palladium could be provided in another form, such as an inorganic palladium salt.

Specific examples disclosed herein have been set forth for the purpose of illustrating applicant's preferred mode and it should be understood that other variations and modifications may be resorted to within the scope and spirit of the claims which follow. 

We claim:
 1. A method for producing uniformly colored, neutral gray stain decorations in glass-ceramic articles which include inhomogeneities which affect stain coloration, comprising the steps of:forming a glassy article from a crystallizable glass composition comprising SiO₂, Al₂ O₃, Li₂ O, ZnO, and as a crystal growth nucleating agent, either TiO₂ or a mixture of TiO₂ and ZrO₂ ; heating said glassy article to a temperature at which a major portion of the article crystallizes within a glassy matrix, thereby forming a glass-ceramic; heating a quantity of palladium-containing stain-decorating composition separately from the glass-ceramic article to a temperature at which the stain-decorating composition is converted to a palladium-containing residue characterized by an ability to produce neutral gray stains when applied to previously crystallized glass-ceramic articles; mixing said palladium-containing residue with a liquid vehicle and applying the mixture to the surface of the glass-ceramic article; and heating the crystallized glass-ceramic article with the decorating mixture thereon to a temperature sufficient to migrate palladium ions from the mixture into surface portions of the article, thereby producing a uniformly colored, neutral gray stained pattern on the article.
 2. The method of claim 1 wherein the glassy article is formed from a crystallizable glass composition comprising:

    ______________________________________                                                         weight percent                                                 ______________________________________                                         SiO.sub.2         67-71                                                        Al.sub.2 O.sub.3  18-21                                                        TiO.sub.2         1.4-5.0                                                      ZrO.sub.2         0-2.0                                                        Sb.sub.2 O.sub.3  0-1.0                                                        As.sub.2 O.sub.5  0-1.0                                                        Li.sub.2 O        2.5-4.0                                                      Na.sub.2 O        0-1.0                                                        K.sub.2 O         0-1.0                                                        Cl.sub.2          0-0.2                                                        ZnO               0.5-2.0                                                      F.sub.2           0-0.5                                                        ______________________________________                                    


3. The method of claim 2 wherein the temperature and total time of the heat treatments are controlled so that the article is transformed to comprise predominantly beta-spodumene throughout the body of the article.
 4. The method of claim 1 or 3 wherein the step of heating the glassy article to crystallize the article is stopped before the crystal structure has attained its final form, and thereafter completing the transformation to the final crystal structure during the final, stain-producing heat treatment.
 5. The method of claim 1 wherein palladium in said stain-decorating composition is present in the form of an organic compound, and said heating of the stain-decorating composition decomposes the palladium organic compound.
 6. The method of claim 1 or 5 wherein the step of heating the stain-decorating composition is carried out at a temperature of at least 1850° F. (1010° C.).
 7. The method of claim 1 wherein the step of mixing the palladium-containing residue comprises mixing the residue with an oil.
 8. The method of claim 1 or 7 wherein the application of the staining mixture onto the article is carried out by silk-screening.
 9. The method of claim 1 wherein the staining mixture applied onto the article includes titanium dioxide.
 10. The method of claim 1 or 9 wherein the staining mixture applied onto the article includes bismuth. 